As mobile technology advances and demand for mobile devices increases, demand for the secondary batteries as energy sources increases rapidly. Among the secondary batteries, the secondary lithium battery has been widely used since its commercialization, due to excellent energy density and voltages, long cycle life, and low rate capability of self discharging.
Further, use of secondary lithium batteries as a driving source of electric vehicle and/or hybrid electric vehicle are actively researched, as environmental concern increases and the consumer's interest increases on the electric vehicles and/or hybrid electric vehicles to replace conventional vehicles run on fossil fuels such as gasoline vehicles or diesel vehicles which are one of the main causes of air pollution.
Meanwhile, the “secondary lithium battery” refers to a battery which includes an electrode assembly including a positive electrode comprising a positive electrode active material allowing intercalation/de-intercalation of lithium ions, a negative electrode comprising negative electrode active material allowing intercalation/de-intercalation of lithium ions, and a microporous separator interposed between the positive and negative electrodes, in which non-aqueous electrolyte comprising lithium ions is contained.
The positive electrode active material of the secondary lithium battery includes a transition metal oxide such as lithium cobalt oxide (LiCoO2), lithium-manganese oxide (LiMn2O4) or lithium-nickel oxide (LiNiO2), and a complex oxide in which part of the transition metals is substituted with other transition metals.
The lithium metal has been used for the negative electrode active material, but in such case, dendrite can form, causing risk of battery short-circuiting and subsequent explosion. Accordingly, lithium metals have recently been replaced by carbon-based materials.
Examples of the carbon-based materials used as the negative electrode active material of the secondary lithium battery include crystalline carbon such as natural graphite and artificial graphite, and amorphous carbon such as soft carbon and hard carbon.
The amorphous carbon has an advantage of high capacity, but also has a disadvantage of high irreversability in charging and discharging process.
The natural graphite, which is a representative example of crystalline carbon, has low cost, excellent initial capacity, and relatively high theoretical marginal capacity, but due to plate-like shape thereof, the natural graphite is oriented to be pressed flat on the current collector when prepared into an electrode plate, thus hindering impregnation of electrolyte, which in turn can severely deteriorate high rate charge/discharge capability, service life, and cycle capacity.
To address the issues mentioned above, it has been suggested to mechanically form the plate-shaped natural graphite into sphere shape for use, or to mix with other graphites. However, possible cracking of the graphite surface or core exposure during rolling can increase side reaction with the electrolyte, which can deteriorate cycle characteristic or swelling characteristic. To make up for the shortcomings mentioned above, some studies suggested a way of using artificial graphite which has better cycle characteristic and swelling characteristic, but at a slightly reduced capacity. However, graphitization is essentially required to use the artificial graphite, and the artificial graphite also has a shortcoming such as higher price than the natural graphite, low resistance to PC-containing electrolyte, and deteriorated output characteristic.
Against this backdrop, in search of a negative electrode active material with superior rollability which can lead into increased density, and superior high rate charging and discharging capability, cycle characteristic and swelling characteristic, the present inventors prepared a secondary graphite particle by aggregating, bonding or assembling an initial natural graphite particle coated with an amorphous carbon material, with an initial artificial graphite particle, and completed the present disclosure by confirming that the secondary lithium battery using the prepared secondary graphite particle as a negative electrode active material exhibited superior high rate charging and discharging capability, cycle characteristic and swelling characteristic.